As it is generally known already, both in the physical deodorization and refining of edible oils and fats, those products are submitted to an extremely rarified environment (2 through 6 mmHg) for a given time and temperature. During this period, water vapor is injected in the system in order to reduce further the partial pressure of the volatile oil/fat components in the inner atmosphere of the refining equipment. In order that an extremely low absolute pressure can be maintained even with injection of direct steam, the industrial equipment is provided with a vacuum system. The function that system is to continuously remove this vapor and other gases (air and volatiles) so as to maintain the inner atmosphere operating at low absolute pressure.
For the above purpose, as widely practiced in the industry, a set of steam ejectors with intermediate condensation is employed. These are well used with or without coupling to liquid ring vacuum pumps. In those systems, the gases which are basically composed of water vapor, air and volatiles, are initially compressed from the operating pressure of the equipment (2 through 6 mmHg) to a higher pressure (typically 30 through 50 mmHg), at which the water can be condensed at usual temperatures. The equipment employed for this compression is a steam ejector, wherein the kinetic energy of the motive steam drags and increases the internal pressure by mixing with vaporous gases to be treated. Depending on the rate of compression required, one, two or three stages of compression are employed. The output flow of the first ejector still is a mixture of gases, mostly water vapor, at a somewhat higher absolute pressure (30 through 50 mmHg). At this pressure, water can be condensed at the usual temperatures. This is done in direct contact condensers employing cooling water. After most of the water vapor has condensed, the incondensables still saturated with water vapor are pumped again up to atmospheric pressure.
Depending on economic considerations, the pressurization is done employing ejector sets with or without intermediate condensation, vacuum pumps or a combination of both. For the first compression stage (from 2-6 mmHg to 30-50), mechanical compressors are not employed due to the high flows involved.
It should be noted further that the ejector that performs the first compression is the part of the apparatus involving most of the steam consumption in the process of refining edible oils/fats. From the energy use viewpoint, the known equipment is extremely inefficient, since it requires a quantity of motive steam two to five times greater than that of the steam dragged. This inefficiency is aggravated where the temperature of the water available for subsequent condensation is high and, accordingly, also the pressure of that water.
The table below, published in the Journal of American Oil Chemists Society", Nr. 2, vol. 62, page 314, of February 1985, illustrates quite accurately the influence of steam water temperature:
______________________________________ SUCTION LOAD: vapour 102 kg/hr. Vacuum: 2.5 mmHg air 10 kg/hr. vapor 3 bar g pressure: Cooling water temp. 26.degree. C. 16.degree. C. Cooling water flow 76 cu.m./hr. 47 cu.m./hr. Vapour consumption 630 kg/hr. 360 kg/hr. Electr. power cons. 17 kW 12 kW ______________________________________
Another important problem, which is associated with deodorization and particularly with the vacuum system employed for deodorizing, is the resulting environmental pollution, both hydric and atmospheric, which results from the high cooling water flows involved. This water is continuously contaminated with organic substances at a very low concentration. The direct treatment of the liquid effluent has a practically prohibitive cost in view of its high volume.
A partial solution normally employed by refineries is to recirculate this water in cooling towers, which causes an eventual and undesirable generation of odors.
Even though this matter has been widely dealt with in recent publications and at congresses in this specific area, there is still no optimim solution for the problem. The solutions heretofore proposed have questionable efficiency or they are handicapped by higher energy consumptions as compared to the invention. An example of a previous solution is to employ indirect heat exchangers to cool the water in the direct contact condensers, for preventing the delivery of organic substance-contaminated water to the cooling tower. The problem or the inconvenience of odor generation is solved, since the contaminated water is recirculated in a closed circuit. However, the temperature of the water is somewhat higher, and a higher differential would be present in the heat exchange in the indirect exchangers. In a warm weather climate, where the water temperature already warrants exceptionally high steam consumption, the above solution is practically unfeasible or results in too high operational costs.